Knowledge of level in industrial process tanks or vessels has long been required for safe and cost-effective operation of plants. Many technologies exist for making level measurements. These include buoyancy, capacitance, ultrasonic and microwave radar, to name a few. Recent advantages in micropower impulse radar (MIR), also known as ultra-wide band (UWB) radar, in conjunction with advances in equivalent time sampling (ETS), permit development of low power and low cost time domain reflectometry (TDR) devices.
In a TDR instrument, a very fast stream of pulses with a rise time of 500 picoseconds, or less, is propagated down a transmission line that serves as a probe in a vessel. The pulses are reflected by a discontinuity caused by a change in impedance, such as at a transition between two media. For level measurement, that transition is typically where the air and the material to be measured meet. Alternatively, the transition could be two different liquids. The amplitude of the reflected signal depends on the difference between the dielectrics of the two media. The dielectric of air is one, while the dielectric of water is about eighty. The larger the difference in dielectric, the larger the reflected signal.
McEwan, U.S. Pat. No. 5,345,471, and other related patents, describe a technique to transmit and receive very fast pulses with simple, low cost and low power electronics. More particularly, McEwan, U.S. Pat. No. 5,609,059, describes a level sensor utilizing this technology. However, the device described therein is intended to be used for simple commercial level applications, such as automobile engine fluid levels. It does not utilize the feature set, power consumption, and versatility required for use in the industrial process environment. Other known devices utilize this technology for a two-wire transmitter using just two wires for both receiving power from the user and sending level information to the user, However, these devices are analog devices limited in the ability to measure the level of extremely low dielectric materials, or materials that coat, clump or build up on the probe, over the wide temperature extremes of industrial process level environments. They also have limited level range capability.
Guided wave radar is one technique available to measure the level of liquids or solids in an industrial environment using TDR principles. Guided wave radar works by generating a stream of pulses of electromagnetic energy and propagating the pulses down a transmission line formed into a level sensing probe. The probe is generally placed vertically in a tank or other container and the electromagnetic pulse is launched downward from the top of the probe. The probe is open to both the air and the material to be sensed in such a way that the electromagnetic fields of the propagating pulse penetrate the air until they reach the level of the material. At that point, the electromagnetic fields see the higher dielectric of the material. This higher dielectric causes a reduction in the impedance of the transmission line, resulting in a pulse echo being reflected back to the top of the probe. The pulse travels through the air dielectric portion of the probe at a known velocity. This allows the material level on the probe to be determined by measuring the round trip travel time of the pulse from the top of the probe to the level and back to the top of the probe. Conductive materials generate echoes similar to the echoes from high dielectric materials. Therefore, the same measurement technique also works with conductive materials.
Guided wave radar measurement instruments measure the time of flight from a known location, referred to as a fiducial, at the top of the probe to the surface of the material of interest in which the probe is immersed. The time of flight is used to calculate distance based on knowledge of the velocity of propagation of the radar pulse through the atmosphere above the surface of the material. Such a measurement instrument is calibrated in room air to determine the effective velocity of propagation. In most applications the velocity of propagation under process conditions differs negligibly from room air. However, in some applications, notably high pressure steam and hydrocarbons, the actual velocity of propagation through the vapor phase differs substantially from the calibrated velocity. This difference can introduce significant error into the distance calculation. Moreover, the propagation velocity may vary in time as process conditions change.
When the process also involves high temperature, the apparent position of the fiducial shifts as the temperature of the solid materials between the fiducial and process vapor rise. This increases the measured time of flight.
The present invention is directed to overcoming one or more of the problems discussed above, in a novel and simple manner.